Computationally efficient technique for determining electrode current distribution from a virtual multipole

ABSTRACT

A system and method of providing therapy to a patient using a plurality of electrodes implanted within the patient. A virtual multipole configuration is defined relative to the plurality of electrodes. The distance between each of a group of the electrodes and a virtual pole of the virtual multipole configuration is determined. A stimulation amplitude distribution is determined for the electrode group based on the determined distances, thereby emulating the virtual multipole configuration. Electrical energy is conveyed from the electrode group in accordance with the computed stimulation amplitude distribution.

RELATED APPLICATION DATA

The present application claims the benefit under 35 U.S.C. §119 to U.S.provisional patent application Ser. No. 61/577,582, filed Dec. 19, 2011.The foregoing application is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF THE INVENTION

The present inventions relate to tissue stimulation systems, and moreparticularly, to neurostimulation systems for programmingneurostimulation leads.

BACKGROUND OF THE INVENTION

Implantable neurostimulation systems have proven therapeutic in a widevariety of diseases and disorders. Pacemakers and Implantable CardiacDefibrillators (ICDs) have proven highly effective in the treatment of anumber of cardiac conditions (e.g., arrhythmias). Spinal CordStimulation (SCS) systems have long been accepted as a therapeuticmodality for the treatment of chronic pain syndromes, and theapplication of tissue stimulation has begun to expand to additionalapplications such as angina pectoralis and incontinence. Deep BrainStimulation (DBS) has also been applied therapeutically for well over adecade for the treatment of refractory chronic pain syndromes, and DBShas also recently been applied in additional areas such as movementdisorders and epilepsy. Further, in recent investigations, PeripheralNerve Stimulation (PNS) and Peripheral Nerve Field Stimulation (PNFS)systems have demonstrated efficacy in the treatment of chronic painsyndromes and incontinence, and a number of additional applications arecurrently under investigation. Furthermore, Functional ElectricalStimulation (FES) systems, such as the Freehand system by NeuroControl(Cleveland, Ohio), have been applied to restore some functionality toparalyzed extremities in spinal cord injury patients.

These implantable neurostimulation systems typically include one or moreelectrode carrying stimulation leads, which are implanted at the desiredstimulation site, and a neurostimulator (e.g., an implantable pulsegenerator (IPG)) implanted remotely from the stimulation site, butcoupled either directly to the neurostimulation lead(s) or indirectly tothe neurostimulation lead(s) via a lead extension. The neurostimulationsystem may further comprise an external control device to remotelyinstruct the neurostimulator to generate electrical stimulation pulsesin accordance with selected stimulation parameters.

Electrical stimulation energy may be delivered from the neurostimulatorto the electrodes in the form of an electrical pulsed waveform. Thus,stimulation energy may be controllably delivered to the electrodes tostimulate neural tissue. The combination of electrodes used to deliverelectrical pulses to the targeted tissue constitutes an electrodecombination, with the electrodes capable of being selectively programmedto act as anodes (positive), cathodes (negative), or left off (zero). Inother words, an electrode combination represents the polarity beingpositive, negative, or zero. Other parameters that may be controlled orvaried include the amplitude, width, and rate of the electrical pulsesprovided through the electrode array. Each electrode combination, alongwith the electrical pulse parameters, can be referred to as a“stimulation parameter set.”

With some neurostimulation systems, and in particular, those withindependently controlled current or voltage sources, the distribution ofthe current to the electrodes (including the case of theneurostimulator, which may act as an electrode) may be varied such thatthe current is supplied via numerous different electrode configurations.In different configurations, the electrodes may provide current orvoltage in different relative percentages of positive and negativecurrent or voltage to create different stimulation amplitudedistributions (i.e., fractionalized electrode combinations).

As briefly discussed above, an external control device can be used toinstruct the neurostimulator to generate electrical stimulation pulsesin accordance with the selected stimulation parameters. Typically, thestimulation parameters programmed into the neurostimulator can beadjusted by manipulating controls on the external control device tomodify the electrical stimulation provided by the neurostimulator systemto the patient. Thus, in accordance with the stimulation parametersprogrammed by the external control device, electrical pulses can bedelivered from the neurostimulator to the stimulation electrode(s) tostimulate or activate a volume of tissue in accordance with a set ofstimulation parameters and provide the desired efficacious therapy tothe patient. The best stimulus parameter set will typically be one thatdelivers stimulation energy to the volume of tissue that must bestimulated in order to provide the therapeutic benefit (e.g., treatmentof pain), while minimizing the volume of non-target tissue that isstimulated.

However, the number of electrodes available, combined with the abilityto generate a variety of complex stimulation pulses, presents a hugeselection of stimulation parameter sets to the clinician or patient. Forexample, if the neurostimulation system to be programmed has an array ofsixteen electrodes, millions of stimulation parameter sets may beavailable for programming into the neurostimulation system. Today,neurostimulation system may have up to thirty-two electrodes, therebyexponentially increasing the number of stimulation parameters setsavailable for programming.

To facilitate such selection, the clinician generally programs theneurostimulator through a computerized programming system. Thisprogramming system can be a self-contained hardware/software system, orcan be defined predominantly by software running on a standard personalcomputer (PC). The PC or custom hardware may actively control thecharacteristics of the electrical stimulation generated by theneurostimulator to allow the optimum stimulation parameters to bedetermined based on patient feedback or other means and to subsequentlyprogram the neurostimulator with the optimum stimulation parameter setor sets, which will typically be those that stimulate all of the targettissue in order to provide the therapeutic benefit, yet minimizes thevolume of non-target tissue that is stimulated. The computerizedprogramming system may be operated by a clinician attending the patientin several scenarios.

For example, in order to achieve an effective result from SCS, the leador leads must be placed in a location, such that the electricalstimulation will cause paresthesia. The paresthesia induced by thestimulation and perceived by the patient should be located inapproximately the same place in the patient's body as the pain that isthe target of treatment. If a lead is not correctly positioned, it ispossible that the patient will receive little or no benefit from animplanted SCS system. Thus, correct lead placement can mean thedifference between effective and ineffective pain therapy. Whenelectrical leads are implanted within the patient, the computerizedprogramming system, in the context of an operating room (OR) mappingprocedure, may be used to instruct the neurostimulator to applyelectrical stimulation to test placement of the leads and/or electrodes,thereby assuring that the leads and/or electrodes are implanted ineffective locations within the patient.

Once the leads are correctly positioned, a fitting procedure, which maybe referred to as a navigation session, may be performed using thecomputerized programming system to program the external control device,and if applicable the neurostimulator, with a set of stimulationparameters that best addresses the painful site. Thus, the navigationsession may be used to pinpoint the volume of activation (VOA) or areascorrelating to the pain. Such programming ability is particularlyadvantageous for targeting the tissue during implantation, or afterimplantation should the leads gradually or unexpectedly move that wouldotherwise relocate the stimulation energy away from the target site. Byreprogramming the neurostimulator (typically by independently varyingthe stimulation energy on the electrodes), the volume of activation(VOA) can often be moved back to the effective pain site without havingto re-operate on the patient in order to reposition the lead and itselectrode array. When adjusting the volume of activation (VOA) relativeto the tissue, it is desirable to make small changes in the proportionsof current, so that changes in the spatial recruitment of nerve fiberswill be perceived by the patient as being smooth and continuous and tohave incremental targeting capability.

One known computerized programming system for SCS is called the BionicNavigator®, available from Boston Scientific NeuromodulationCorporation. The Bionic Navigator® is a software package that operateson a suitable PC and allows clinicians to program stimulation parametersinto an external handheld programmer (referred to as a remote control).Each set of stimulation parameters, including fractionalized currentdistribution to the electrodes (as percentage cathodic current,percentage anodic current, or off), may be stored in both the BionicNavigator® and the remote control and combined into a stimulationprogram that can then be used to stimulate multiple regions within thepatient.

Prior to creating the stimulation programs, the Bionic Navigator® may beoperated by a clinician in a “manual mode” to manually select thepercentage cathodic current and percentage anodic current flowingthrough the electrodes, or may be operated by the clinician in an“automated mode” to electrically “steer” the current along the implantedleads in real-time (e.g., using a joystick or joystick-like controls),thereby allowing the clinician to determine the most efficaciousstimulation parameter sets that can then be stored and eventuallycombined into stimulation programs. In the context of SCS, currentsteering is typically either performed in a rostro-caudal direction(i.e., along the axis of the spinal cord) or a medial-lateral direction(i.e., perpendicular to the axis of the spinal cord).

In one novel method, described in U.S. patent application Ser. No.12/938,282, entitled “System and Method for Mapping Arbitrary ElectricFields to Pre-existing Lead Electrodes,” which is incorporated herein byreference, a stimulation target in the form of an ideal (or virtual)target multipole pole (e.g., a virtual bipole or tripole) is defined andthe stimulation parameters, including the stimulation amplitudedistribution on the electrodes, are computationally determined in amanner that emulates these virtual multipoles. This technique involvesdefining the virtual multipole, computing field potential values thatthe virtual multipole creates on an array of spatial observation points,and determining the stimulation amplitude distribution on the electrodesthat would result in estimated electrical field potential values at thespatial observation points that best matches the desired field potentialvalues at the spatial observation points. It can be appreciated thatcurrent steering can be implemented by moving the virtual multipolesabout the leads, such that the appropriate stimulation amplitudedistribution for the electrodes is computed for each of the variouspositions of the virtual multipole. As a result, the current steeringcan be implemented using an arbitrary number and arrangement ofelectrodes.

While the computation of the stimulation amplitude distribution on theelectrodes to emulate a virtual multipole is quite useful, acomputationally intensive optimization technique utilizing an inversetransfer matrix is used to determine the stimulation amplitudedistribution, which may not present a significant issue if thecomputerized programming system has the necessary computational power,but may present a significant issue if the optimization technique isincorporated into a less computationally powerful device, such as aremote control. Furthermore, the computation of the stimulationamplitude distribution on the electrodes may result in electrodes withsmall percentages of current that do not actively contribute to theelectrical stimulation field. A “cleaning algorithm” may be used to nullout the electrodes with current percentages below a specified threshold.However, this cleaning algorithm may not provide a smooth transition ofthe virtual multipole in space during current steering.

There, thus, remains a need for a more computationally efficienttechnique for determining the stimulation parameters that best emulate avirtual multipole.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present inventions, a systemfor an electrical neurostimulator coupled to a plurality of electrodesis provided. The processor is configured for defining a virtualmultipole configuration (e.g., a virtual bipole configuration or avirtual tripole configuration) relative to the plurality of electrodes,and determining a distance between each of a group of the electrodes anda virtual pole (e.g., a virtual cathode or a virtual anode) of thevirtual multipole configuration.

In one embodiment, the processor is further configured for limiting thenumber of electrodes in the electrode group to a maximum number. In thiscase, the processor may further be configured for varying the maximumnumber of the electrodes in the electrode group in response to a userinput. In another embodiment, the processor is further configured forcomparing a function of a distance (e.g., a proportional function, ahigher order power function, or an exponential function) between each ofthe plurality of electrodes and the virtual pole to a threshold value,and excluding the electrode from the electrode group based on thecomparison.

The processor is further configured for determining a stimulationamplitude distribution for the electrode group based on the determineddistances, thereby emulating the virtual multipole configuration. In oneembodiment, the processor is configured for determining the stimulationamplitude distribution for the electrode group by computing weights asdecreasing functions of the distances (e.g., an inverse proportionalfunction of the respective distance, an inverse higher order powerfunction of the respective distance, or an inverse exponential functionof the respective distance), and computing stimulation amplitude valuesrespectively for the electrode group as a function of the computedweights. The stimulation amplitude values may be fractionalized currentvalues, in which case, the processor may be configured for computing afractionalized current value for each electrode of the electrode groupas the quotient of the weight for the each electrode divided by the sumof the weights for the electrode group.

The system further comprises a controller configured for instructing theelectrical neurostimulator to convey electrical energy to the electrodegroup in accordance with the computed stimulation amplitudedistribution. The system optionally comprises a user interfaceconfigured for generating directional control signals, in which case,the processor may be further configured for modifying the virtualmultipole configuration relative to the plurality of electrodes inresponse to the directional control signals, and repeating the distancedetermination and stimulation amplitude distribution steps for themodified virtual multipole configuration, and the controller may befurther configured for repeating the electrical energy conveyance stepfor the modified virtual multipole configuration. The system may furthercomprise telemetry circuitry, in which case, the controller may furtherbe configured for transmitting stimulation parameter sets defining thecomputed stimulation amplitude distribution to the neurostimulationdevice via the telemetry circuitry. The system may also comprise ahousing containing the processor and controller.

In accordance with a second aspect of the present inventions, a methodof providing therapy to a patient using a plurality of electrodesimplanted within the patient is provided. The method comprises defininga virtual multipole configuration (e.g., a virtual bipole configurationor a virtual tripole configuration) relative to the plurality ofelectrodes, and determining a distance between each of a group ofelectrodes and a virtual pole (e.g., a virtual cathode or a virtualanode) of the virtual multipole configuration.

One method comprises limiting the number of electrodes in the electrodegroup to a maximum number. In this case, the maximum number of theelectrodes in the electrode group may be varied in response to a userinput. Another method comprises comparing a function of a distance(e.g., a proportional function, a higher order power function, or anexponential function) between each of the plurality of electrodes andthe virtual pole to a threshold value, and excluding the electrode fromthe electrode group based on the comparison.

The method further comprises determining a stimulation amplitudedistribution for the electrode group based on the determined distances,thereby emulating the virtual multipole configuration. The stimulationamplitude distribution for the electrode group may be determined bycomputing weights as decreasing functions of the distances (e.g., aninverse proportional function of the respective distance, an inversehigher order power function of the respective distance, or an inverseexponential function of the respective distance), and computingstimulation amplitude values respectively for the electrode group as afunction of the computed weights. The stimulation amplitude values maybe fractionalized current values, in which case, a fractionalizedcurrent value can be computed for each electrode of the electrode groupas the quotient of the weight for the each electrode divided by the sumof the weights for the electrode group.

The method further comprises conveying electrical energy from theelectrode group in accordance with the computed stimulation amplitudedistribution. An optional method comprises generating directionalcontrol signals, modifying the virtual multipole configuration relativeto the plurality of electrodes in response to the directional controlsignals, and repeating the distance determination, stimulation amplitudedistribution, and electrical energy conveyance steps for the modifiedvirtual multipole configuration.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a plan view of a Spinal cord Stimulation (SCS) systemconstructed in accordance with one embodiment of the present inventions;

FIG. 2 is a perspective view of the arrangement of the SCS system ofFIG. 1 with respect to a patient;

FIG. 3 is a profile view of an implantable pulse generator (IPG) and asurgical paddle lead used in the SCS system of FIG. 1;

FIG. 4 is a profile view of an implantable pulse generator (IPG) andpercutaneous leads used in the SCS system of FIG. 1;

FIG. 5 is front view of a remote control (RC) used in the SCS system ofFIG. 1;

FIG. 6 is a block diagram of the internal components of the RC of FIG.4;

FIG. 7 is a block diagram of the internal components of a clinician'sprogrammer (CP) used in the SCS system of FIG. 1;

FIG. 8 is a plan view of a programming screen generated by the CP ofFIG. 7 for programming the IPG of FIG. 3;

FIG. 9 is a flow diagram illustrating one method of emulating a virtualmultipole configuration generated in the programming screen of FIG. 8;

FIG. 10 is a plan view of an exemplary virtual tripole configurationdefined relative to an electrode array; and

FIGS. 11 a and 11 b are plan views of the virtual cathode of the virtualtripole configuration defined relative to a group of electrodes used toemulate the virtual cathode.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a spinal cord stimulation (SCS)system. However, it is to be understood that while the invention lendsitself well to applications in SCS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, peripheral nerve stimulator, microstimulator,or in any other neurostimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 1, an exemplary SCS system 10 generally comprisesat least one implantable neurostimulation lead 12, an implantable pulsegenerator (IPG) 14 (or alternatively RF receiver-stimulator), anexternal remote control RC 16, a Clinician's Programmer (CP) 18, anExternal Trial Stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more lead extensions 24 tothe neurostimulation lead 12, which carries a plurality of electrodes 26arranged in an array. The neurostimulation lead 12 is illustrated as asurgical paddle lead in FIG. 1, although as will be described in furtherdetail below, one or more percutaneous stimulation leads can be used inplace of the surgical paddle lead 12. As will also be described infurther detail below, the IPG 14 includes pulse generation circuitrythat delivers electrical stimulation energy in the form of a pulsedelectrical waveform (i.e., a temporal series of electrical pulses) tothe electrode array 26 in accordance with a set of stimulationparameters.

The ETS 20, which has similar pulse generation circuitry as the IPG 14,also provides electrical stimulation energy to the electrode array 26 inaccordance with a set of stimulation parameters. The major differencebetween the ETS 20 and the IPG 14 is that the ETS 20 is anon-implantable device that is used on a trial basis after theneurostimulation leads 12 have been implanted and prior to implantationof the IPG 14, to test the responsiveness of the stimulation that is tobe provided. Thus, any functions described herein with respect to theIPG 14 can likewise be performed with respect to the ETS 20. Furtherdetails of an exemplary ETS are described in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation lead 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation programs after implantation. Oncethe IPG 14 has been programmed, and its power source has been charged orotherwise replenished, the IPG 14 may function as programmed without theRC 16 being present.

The CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions. The CP 18 may perform this function by indirectlycommunicating with the IPG 14 or ETS 20, through the RC 16, via an IRcommunications link 36. Alternatively, the CP 18 may directlycommunicate with the IPG 14 or ETS 20 via an RF communications link (notshown).

The external charger 22 is a portable device used to transcutaneouslycharge the IPG 14 via an inductive link 38. For purposes of brevity, thedetails of the external charger 22 will not be described herein. Detailsof exemplary embodiments of external chargers are disclosed in U.S. Pat.No. 6,895,280, which has been previously incorporated herein byreference. Once the IPG 14 has been programmed, and its power source hasbeen charged by the external charger 22 or otherwise replenished, theIPG 14 may function as programmed without the RC 16 or CP 18 beingpresent.

As shown in FIG. 2, the neurostimulation lead 12 is implanted within thespinal column 42 of a patient 40. The preferred placement of theneurostimulation lead 12 is adjacent, i.e., resting upon, the spinalcord area to be stimulated. Due to the lack of space near the locationwhere the neurostimulation leads 12 exit the spinal column 42, the IPG14 is generally implanted in a surgically-made pocket either in theabdomen or above the buttocks. The IPG 14 may, of course, also beimplanted in other locations of the patient's body. The lead extension24 facilitates locating the IPG 14 away from the exit point of theneurostimulation leads 12. As there shown, the CP 18 communicates withthe IPG 14 via the RC 16.

Referring to FIG. 3, the IPG 14 comprises an outer case 44 for housingthe electronic and other components (described in further detail below),and a connector 46 to which the proximal end of the neurostimulationlead 12 mates in a manner that electrically couples the electrodes 26 tothe internal electronics (described in further detail below) within theouter case 44. The outer case 44 is composed of an electricallyconductive, biocompatible material, such as titanium, and forms ahermetically sealed compartment wherein the internal electronics areprotected from the body tissue and fluids. In some cases, the outer case44 may serve as an electrode.

In the embodiment illustrated in FIG. 3, the neurostimulation lead 12takes the form of a surgical paddle lead 12 on which the electrodes 26(in this case, electrodes E1-E32) are carried. The electrodes 26 arearranged in a two-dimensional array in four columns along the axis ofthe neurostimulation lead 12. In the illustrated embodiment, theelectrodes 26 are arranged in two inner columns of electrodes 26′(electrodes E9-E24), and two outer columns of electrodes 26″ (electrodesE1-E8 and E25-E32) that flank and are longitudinally offset from theinner electrode columns. In other embodiments, the outer and innerelectrode columns may not be longitudinally offset from each other. Theactual number of leads and electrodes will, of course, vary according tothe intended application. Further details regarding the construction andmethod of manufacture of surgical paddle leads are disclosed in U.S.patent application Ser. No. 11/319,291, entitled “Stimulator Leads andMethods for Lead Fabrication,” and U.S. patent application Ser. No.12/204,094, entitled “Multiple Tunable Central Cathodes on a Paddle forIncreased Medial-Lateral and Rostro-Caudal Flexibility via CurrentSteering, the disclosures of which are expressly incorporated herein byreference.

In an alternative embodiment illustrated in FIG. 4, the neurostimulationlead 12 takes the form of a percutaneous stimulation lead on which theelectrodes 12 (in this case, electrodes E1-E16) are disposed as ringelectrodes. In the illustrated embodiment, two percutaneous leads 12 onwhich electrodes E1-E8 and E9-E16 are disposed can be used with the SCSsystem 10. The actual number and shape of leads and electrodes will, ofcourse, vary according to the intended application. Further detailsdescribing the construction and method of manufacturing percutaneousstimulation leads are disclosed in U.S. patent application Ser. No.11/689,918, entitled “Lead Assembly and Method of Making Same,” and U.S.patent application Ser. No. 11/565,547, entitled “CylindricalMulti-Contact Electrode Lead for Neural Stimulation and Method of MakingSame,” the disclosures of which are expressly incorporated herein byreference.

As will be described in further detail below, the IPG 14 includes pulsegeneration circuitry that provides electrical conditioning andstimulation energy in the form of a pulsed electrical waveform to theelectrode array 26 in accordance with a set of stimulation parametersprogrammed into the IPG 14. Such stimulation parameters may compriseelectrode combinations, which define the electrodes that are activatedas anodes (positive), cathodes (negative), and turned off (zero),percentage of stimulation energy assigned to each electrode(fractionalized electrode configurations), and electrical pulseparameters, which define the pulse amplitude (measured in milliamps orvolts depending on whether the IPG 14 supplies constant current orconstant voltage to the electrode array 26), pulse width (measured inmicroseconds), pulse rate (measured in pulses per second), and burstrate (measured as the stimulation on duration X and stimulation offduration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case. Simulation energy may betransmitted to the tissue in a monopolar or multipolar (e.g., bipolar,tripolar, etc.) fashion. Monopolar stimulation occurs when a selectedone of the lead electrodes 26 is activated along with the case of theIPG 14, so that stimulation energy is transmitted between the selectedelectrode 26 and case. Bipolar stimulation occurs when two of the leadelectrodes 26 are activated as anode and cathode, so that stimulationenergy is transmitted between the selected electrodes 26. For example,an electrode on one lead 12 may be activated as an anode at the sametime that an electrode on the same lead or another lead 12 is activatedas a cathode. Tripolar stimulation occurs when three of the leadelectrodes 26 are activated, two as anodes and the remaining one as acathode, or two as cathodes and the remaining one as an anode. Forexample, two electrodes on one lead 12 may be activated as anodes at thesame time that an electrode on another lead 12 is activated as acathode.

The stimulation energy may be delivered between electrodes as monophasicelectrical energy or multiphasic electrical energy. Monophasicelectrical energy includes a series of pulses that are either allpositive (anodic) or all negative (cathodic). Multiphasic electricalenergy includes a series of pulses that alternate between positive andnegative. For example, multiphasic electrical energy may include aseries of biphasic pulses, with each biphasic pulse including a cathodic(negative) stimulation phase and an anodic (positive) recharge phasethat is generated after the stimulation phase to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is delivered through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the recharge pulse).

In the illustrated embodiment, the IPG 14 can individually control themagnitude of electrical current flowing through each of the electrodes.In this case, it is preferred to have a current generator, whereinindividual current-regulated amplitudes from independent current sourcesfor each electrode may be selectively generated. Although this system isoptimal to take advantage of the invention, other stimulators that maybe used with the invention include stimulators having voltage regulatedoutputs. While individually programmable electrode amplitudes areoptimal to achieve fine control, a single output source switched acrosselectrodes may also be used, although with less fine control inprogramming. Mixed current and voltage regulated devices may also beused with the invention. Further details discussing the detailedstructure and function of IPGs are described more fully in U.S. Pat.Nos. 6,516,227 and 6,993,384, which are expressly incorporated herein byreference.

It should be noted that rather than an IPG, the SCS system 10 mayalternatively utilize an implantable receiver-stimulator (not shown)connected to the neurostimulation leads 12. In this case, the powersource, e.g., a battery, for powering the implanted receiver, as well ascontrol circuitry to command the receiver-stimulator, will be containedin an external controller inductively coupled to the receiver-stimulatorvia an electromagnetic link. Data/power signals are transcutaneouslycoupled from a cable-connected transmission coil placed over theimplanted receiver-stimulator. The implanted receiver-stimulatorreceives the signal and generates the stimulation in accordance with thecontrol signals.

Referring now to FIG. 5, one exemplary embodiment of an RC 16 will nowbe described. As previously discussed, the RC 16 is capable ofcommunicating with the IPG 14, CP 18, or ETS 20. The RC 16 comprises acasing 50, which houses internal componentry (including a printedcircuit board (PCB)), and a lighted display screen 52 and button pad 54carried by the exterior of the casing 50. In the illustrated embodiment,the display screen 52 is a lighted flat panel display screen, and thebutton pad 54 comprises a membrane switch with metal domes positionedover a flex circuit, and a keypad connector connected directly to a PCB.In an optional embodiment, the display screen 52 has touch screencapabilities. The button pad 54 includes a multitude of buttons 56, 58,60, and 62, which allow the IPG 14 to be turned ON and OFF, provide forthe adjustment or setting of stimulation parameters within the IPG 14,and provide for selection between screens.

In the illustrated embodiment, the button 56 serves as an ON/OFF buttonthat can be actuated to turn the IPG 14 ON and OFF. The button 58 servesas a select button that allows the RC 16 to switch between screendisplays and/or parameters. The buttons 60 and 62 serve as up/downbuttons that can be actuated to increment or decrement any ofstimulation parameters of the pulse generated by the IPG 14, includingpulse amplitude, pulse width, and pulse rate. For example, the selectionbutton 58 can be actuated to place the RC 16 in a “Pulse AmplitudeAdjustment Mode,” during which the pulse amplitude can be adjusted viathe up/down buttons 60, 62, a “Pulse Width Adjustment Mode,” duringwhich the pulse width can be adjusted via the up/down buttons 60, 62,and a “Pulse Rate Adjustment Mode,” during which the pulse rate can beadjusted via the up/down buttons 60, 62. Alternatively, dedicatedup/down buttons can be provided for each stimulation parameter. Ratherthan using up/down buttons, any other type of actuator, such as a dial,slider bar, or keypad, can be used to increment or decrement thestimulation parameters. Further details of the functionality andinternal componentry of the RC 16 are disclosed in U.S. Pat. No.6,895,280, which has previously been incorporated herein by reference.

Referring to FIG. 6, the internal components of an exemplary RC 16 willnow be described. The RC 16 generally includes a processor 64 (e.g., amicrocontroller), memory 66 that stores an operating program forexecution by the processor 64, as well as stimulation parameter sets ina navigation table (described below), input/output circuitry, and inparticular, telemetry circuitry 68 for outputting stimulation parametersto the IPG 14 and receiving status information from the IPG 14, andinput/output circuitry 70 for receiving stimulation control signals fromthe button pad 54 and transmitting status information to the displayscreen 52 (shown in FIG. 5). As well as controlling other functions ofthe RC 16, which will not be described herein for purposes of brevity,the processor 64 generates new stimulation parameter sets in response tothe user operation of the button pad 54. These new stimulation parametersets would then be transmitted to the IPG 14 via the telemetry circuitry68. Further details of the functionality and internal componentry of theRC 16 are disclosed in U.S. Pat. No. 6,895,280, which has previouslybeen incorporated herein by reference.

As briefly discussed above, the CP 18 greatly simplifies the programmingof multiple electrode combinations, allowing the user (e.g., thephysician or clinician) to readily determine the desired stimulationparameters to be programmed into the IPG 14, as well as the RC 16. Thus,modification of the stimulation parameters in the programmable memory ofthe IPG 14 after implantation is performed by a user using the CP 18,which can directly communicate with the IPG 14 or indirectly communicatewith the IPG 14 via the RC 16. That is, the CP 18 can be used by theuser to modify operating parameters of the electrode array 26 near thespinal cord.

As shown in FIG. 2, the overall appearance of the CP 18 is that of alaptop personal computer (PC), and in fact, may be implanted using a PCthat has been appropriately configured to include adirectional-programming device and programmed to perform the functionsdescribed herein. Alternatively, the CP 18 may take the form of amini-computer, personal digital assistant (PDA), smartphone, etc., oreven a remote control (RC) with expanded functionality. Thus, theprogramming methodologies can be performed by executing softwareinstructions contained within the CP 18. Alternatively, such programmingmethodologies can be performed using firmware or hardware. In any event,the CP 18 may actively control the characteristics of the electricalstimulation generated by the IPG 14 to allow the optimum stimulationparameters to be determined based on patient response and feedback andfor subsequently programming the IPG 14 with the optimum stimulationparameters.

To allow the user to perform these functions, the CP 18 includes a mouse72, a keyboard 74, and a programming display screen 76 housed in a case78. It is to be understood that in addition to, or in lieu of, the mouse72, other directional programming devices may be used, such as atrackball, touchpad, joystick, or directional keys included as part ofthe keys associated with the keyboard 74. In the illustrated embodiment,the monitor 76 is a conventional screen. Alternatively, instead of beingconventional, the monitor 76 may be a digitizer screen, such astouchscreen (not shown), and may be used in conjunction with an activeor passive digitizer stylus/finger touch.

As shown in FIG. 7, the CP 18 further includes a controller/processor 80(e.g., a central processor unit (CPU)) and memory 82 that stores astimulation programming package 84, which can be executed by thecontroller/processor 80 to allow the user to program the IPG 14, and RC16. Notably, while the controller/processor 80 is shown as a singledevice, the processing functions and controlling functions can beperformed by a separate controller and processor. Thus, it can beappreciated that the controlling functions described below as beingperformed by the CP 18 can be performed by a controller, and theprocessing functions described below as being performed by the CP 18 canbe performed by a processor. The CP 18 further includes output circuitry86 (e.g., via the telemetry circuitry of the RC 16) for downloadingstimulation parameters to the IPG 14 and RC 16 and for uploadingstimulation parameters already stored in the memory 66 of the RC 16, viathe telemetry circuitry 68 of the RC 16.

Execution of the programming package 84 by the controller/processor 80provides a multitude of display screens (not shown) that can benavigated through via use of the mouse 72. These display screens allowthe clinician to, among other functions, to select or enter patientprofile information (e.g., name, birth date, patient identification,physician, diagnosis, and address), enter procedure information (e.g.,programming/follow-up, implant trial system, implant IPG, implant IPGand lead(s), replace IPG, replace IPG and leads, replace or reviseleads, explant, etc.), generate a pain map of the patient, define theconfiguration and orientation of the leads, initiate and control theelectrical stimulation energy output by the leads 12, and select andprogram the IPG 14 with stimulation parameters in both a surgicalsetting and a clinical setting. Further details discussing theabove-described CP functions are disclosed in U.S. patent applicationSer. No. 12/501,282, entitled “System and Method for Converting TissueStimulation Programs in a Format Usable by an Electrical CurrentSteering Navigator,” and U.S. patent application Ser. No. 12/614,942,entitled “System and Method for Determining Appropriate Steering Tablesfor Distributing Stimulation Energy Among Multiple NeurostimulationElectrodes,” which are expressly incorporated herein by reference.

Most pertinent to the present inventions, execution of the programmingpackage 84 provides a user interface that conveniently allows a user toprogram the IPG 14 in a manner that emulates an ideal multipole, such asa bipole or tripole, much like in the manner described in U.S. patentapplication Ser. No. 12/938,282, which was previously incorporatedherein by reference. However, in this case, the programming package 84provides a series of different ideal multipole configurations that canbe used to steer electrical current relative to the electrodes 12 inresponse to directional control signals generated in response tomanipulation of a directional programming device, such as one or more ofthe directional programming devices described above.

Referring now to FIG. 8, an exemplary programming screen 100 generatedby the CP 16 to allow a user to program the IPG 14 will now bedescribed. The programming screen 100 includes various control elementsdescribed below that can be actuated to perform various controlfunctions.

A pointing element may be placed on any of the control elements toperform the actuation event. As described above, in the case of adigitizer touch screen, the pointing element will be an actual pointingelement (e.g., a finger or active or passive stylus) that can be used tophysically tap the screen above the respective graphical control elementor otherwise brought into proximity with respect to the graphicalcontrol element. In the case of a conventional screen, the pointingelement will be a virtual pointing element (e.g., a cursor) that can beused to graphically click on the respective control element.

The programming screen 100 includes an electrode combination control 102having arrows that can be actuated by the user to select one of fourdifferent electrode combinations 1-4. The programming screen 100 furtherincludes a stimulation on/off control 104 that can be alternatelyactuated initiate or cease the delivery of electrical stimulation energyfrom the IPG 14 via the selected electrode combination.

The programming screen 100 further includes various stimulationparameter controls that can be operated by the user to manually adjuststimulation parameters for the selected electrode combination. Inparticular, the programming screen 100 includes a pulse width adjustmentcontrol 106 (expressed in microseconds (μs)), a pulse rate adjustmentcontrol 108 (expressed in Hertz (Hz)), and a pulse amplitude adjustmentcontrol 110 (expressed in milliamperes (mA)). Each control includes afirst arrow that can be actuated to decrease the value of the respectivestimulation parameter and a second arrow that can be actuated toincrease the value of the respective stimulation parameter.

The programming screen 100 also includes a set of axial steering controlelements 112 and a set of transverse steering control elements 114. Inthe illustrated embodiments, the control elements 112, 114, as well asthe other control elements discussed herein, are implemented as agraphical icon that can be clicked with a mouse or touched with a fingerin the case of a touchscreen. Alternatively, the control elementsdescribed herein may be implemented as a joy stick, touchpad, buttonpad, group of keyboard arrow keys, mouse, roller ball tracking device,horizontal or vertical rocker-type arm switches, etc., that can bepressed or otherwise moved to actuate the control elements.

When any of the axial steering control elements 112 is actuated, controlsignals are generated in response to which the controller/processor 80is configured for generating stimulation parameter sets designed toaxially displace the locus of the electrical stimulation field (andthus, the volume of activation (VOA)) relative to the axis of the lead12. Likewise, when any of the transverse steering control elements 114is actuated, control signals are generated in response to which thecontroller/processor 80 is configured for generating stimulationparameter sets designed to transversely displace the locus of theelectrical stimulation field (and thus, the VOA) relative to the axis ofthe lead 12.

The control elements 112, 114 may be continually actuated (i.e., bycontinuously actuating one of the control elements 112, 114, e.g., byclicking on one of the control elements 112, 114 and holding the click(i.e., continuous actuation of the control following the initial“click”), or repeatedly actuating one of the control elements 112, 114,e.g., by repeatedly clicking and releasing one of the control elements112, 114) to generate a series of control signals in response to whichthe controller/processor 80 is configured for generating the pluralityof stimulation parameter sets. The output telemetry circuitry 86 isconfigured for transmitting these stimulation parameters sets to the IPG14.

Preferably, the control signals that are generated in response to theactuation of the control elements 112, 114 or the alternative controlelements are directional, meaning that the locus of the electricalstimulation field will be displaced in a defined direction in responseto a continual actuation of a single control element irrespective of thecurrent position of the locus electrical stimulation field locus. Aswill be described in further detail below, the controller/processor 80,in response to the actuation of the control elements 112, 114, firstdefines a series of ideal multipoles, and computationally determines thestimulation parameters, including the fractional ized current values oneach of the electrodes, in a manner that emulates these idealmultipoles.

Each of the sets of control elements 112, 114 takes the form of a doublearrow (i.e., two oppositely pointing control element arrows) that can beactuated to modify the electrical stimulation field depending on themode of operation. For example, an upper arrow control element 112 a canbe clicked to axially displace (i.e., along the axis of the lead 12) thelocus of the electrical stimulation field in the proximal direction; alower arrow control element 112 b can be clicked to axially displace(i.e., along the axis of the lead 12) the locus of the electricalstimulation field in the distal direction; a left arrow control element114 a can be clicked to transversely displace (i.e., perpendicular tothe axis of the lead 12) the locus of the electrical stimulation fieldin the leftward direction; and a right arrow control element 114 b canbe clicked to transversely displace (i.e., perpendicular to the axis ofthe lead 12) the locus of the electrical stimulation field in therightward direction. The control elements 112, 114 also includeindicators 112 c, 114 c for displaying an indication of the locus of theelectrical stimulation field relative to the lead 12. In particular, anindicator 112 c displays a dot representative of the axial displacementof the electrical stimulation field locus, and an indicator 114 cdisplays a dot representative of the transverse displacement of theelectrical stimulation field locus.

The programming screen 100 displays graphical representations of theelectrodes 26′. In the illustrated embodiment, each electroderepresentation 26′ takes the form of a closed geometric figure, and inthis case a rectangle. In alternative embodiments, the electroderepresentations 26′ can take the form of other types of closed geometricfigures, such as circles. The programming screen 100 also displays agraphical rendering of an ideal multipole 150 relative to the electrodearray 26.

In one embodiment, the multipole 150 is a generalized multipole thatdefines five imaginary locations for a central virtual pole 152 and foursurrounding virtual poles 154(1)-154(4) (collectively, 154). Thegeneralized multipole 150 is defined with several sets of variablevalues that are stored in memory 82. These sets of variable valuesinclude a set of variable values defining the polarities of the centralvirtual pole 152 and surrounding virtual poles 154, a set of variablevalues defining a spatial relationship between the central virtual pole152 and the electrode array 26, a set of variable values defining aspatial relationship between the surrounding virtual poles 154 and thecentral virtual pole 152, and a set of variable values defining relativeintensities of the surrounding virtual poles 154. In practice, the setof variable values defining the relative intensities of the surroundingvirtual poles 154 will typically be defined, such that a tripole or evena bipole configuration results. For example, a virtual tripoleconfiguration arranged in a rostro-caudal manner (vertical tripole) canbe defined by setting the intensities for the virtual poles 154(1) and154(2) to a non-zero value, while setting the intensities for thevirtual poles 154(3) and 154(4) to a zero value. A virtual tripoleconfiguration arranged in a medio-lateral manner (horizontal tripole)can be defined by setting the intensities for the virtual poles 154(1)and 154(2) to a zero value, while setting the intensities for thevirtual poles 154(3) and 154(4) to a non-zero value.

The ideal multipole 150 may be manipulated by the controller/processor80 relative to the electrode array 26 in response to the control signalsgenerated by actuation of the steering control elements 112, 114.Alternatively, rather than automatically manipulating the idealmultipole 150 in response to the current steering control signals, theideal multipole 150 may be manually manipulated, e.g., by touching anyof the virtual poles with a physical pointing device or otherwiseclicked with a virtual pointing device and dragged to a differentlocation relative to the electrode representations 26′. For example, thecentral virtual pole 152 may be dragged to displace the entire idealmultiple 150 relative to the electrode representations 26′ or one of thesurrounding virtual poles 154 may be dragged to displace it relative tothe central virtual pole 152.

Further details discussing techniques for using a generalized virtualmultipole to define various types of multipoles, as well specifictechniques for steering current using ideal multipole configurations,are described in U.S. Provisional Patent Application Ser. No.61/452,965, entitled “Neurostimulation System for Defining A GeneralizedIdeal Multipole Configuration,” which is expressly incorporated hereinby reference.

However the virtual multipole configuration is manipulated relative tothe electrode array 26, the controller/processor 80 is configured fordetermining the stimulation amplitude distribution for the electrodearray 26 that emulates the virtual multipole configuration in acomputationally efficient manner.

Having described the structure and function of the neurostimulation 10,one method of providing therapy to a patient utilizing the programmerscreen 100 shown in FIG. 8 will now be described with reference to FIG.9. First, the user modifies the virtual multipole configuration 150(step 200). For example, the user may manipulate the current steeringcontrol elements 112, 114 to automatically modify the virtual multipoleconfiguration, or may alternatively, drag the virtual poles to manuallymodify the virtual multipole configuration. The controller/processor 80then defines the modified virtual multipole configuration relative tothe electrode array 26 (step 202).

In an example shown in FIG. 10, a vertically oriented virtual tripoleconfiguration 160 is formed from the variable generalized idealmultipole configuration 150. The virtual tripole configuration 160 isshown defined relative to the electrode array 26, and has a centralvirtual cathode 162, an upper virtual anode 164, and a lower virtualanode 166. The size of each of the poles in the virtual tripoleconfiguration is indicative of the magnitude of current assigned to therespective poles. In the illustrated embodiment, all of the cathodiccurrent is assigned to the central virtual cathode 162, and more of theanodic current is assigned to the upper virtual anode 164 than the lowervirtual anode 166 (e.g., 75% of the anodic current can be assigned tothe upper virtual anode 164 and 25% of the anodic current can beassigned to the lower virtual anode 166).

Next, the controller/processor 80 selects a pole of the virtualmultipole configuration 150 to be emulated (step 204), determines adistance between each of the electrodes 26 and the selected virtual pole(step 206), and based on these determined distances, selects certainones of these electrodes 26 to be included within a group of theelectrodes 26 that will emulate the selected virtual pole (step 208).Because the controller/processor 80 has already defined the virtualmultipole configuration 150 relative to the electrode array 26, thecontroller/processor 80 need only perform simple coordinate systemcomputations in determining the distances between the electrodes 26 andeach of the virtual poles.

In an optional embodiment, a clustering algorithm can be used todetermine electrodes that are in close proximity to each other. This maybe useful in the case where electrode clusters are separated from eachother by a relatively long distance, such that electrodes from differentclusters would not be selected to emulate a particular virtual pole.Rather, only electrodes from the one cluster that is closest inproximity to the virtual pole should be selected to be included withinthe electrode group that emulates the virtual pole. In this case, theelectrodes that will be ultimately used to emulate the virtual pole willonly be selected from this electrode cluster.

In the illustrated embodiment, the number of electrodes in the group islimited to a maximum number (e.g., four), although the number ofelectrodes in the group may be selected to be less than the maximumnumber (e.g., if a virtual pole is located directly in the center of anelectrode, only one electrode is needed to emulate that electrode, or ifless than the maximum number of electrodes are adjacent the virtualpole). This maximum number can be fixed or can be varied by the user(e.g., via a control element (not shown) in the programming screen 100)in order to control the focus/blur of the resulting stimulation. Inanother embodiment, there is no maximum number of electrodes in thegroup, but rather, the number of electrodes in the group may bevariable. For example, if the density of the electrodes adjacent thevirtual pole is relatively high, more electrodes can be selected for thegroup. On the contrary, if the density of the electrodes adjacent thevirtual pole is relatively low, less electrodes can be selected for thegroup.

Preferably, the electrodes that are closest to the virtual pole to beemulated are selected to be included within the electrode group. Forexample, as illustrated in FIG. 10, the four electrodes E10, E11, E18,and E19 are closest in proximity to the upper virtual anode 164, and arethus, selected to be included within an electrode group for the uppervirtual anode 164; the four electrodes E12, E13, E20, and E21 areclosest in proximity to the central virtual cathode 162, and are thus,selected to be included within an electrode group for the centralvirtual cathode 162; and the four electrodes E14, E15, E22, and E23 areclosest in proximity to the lower virtual anode 166, and are thus,selected to be included within an electrode group for the lower virtualanode 166.

When determining the electrodes to be included within a group, thecontroller/processor 80 may optionally compare a function of a distancebetween each of the electrodes 26 and the relevant virtual pole to athreshold value, and then exclude certain ones of the electrodes 26 fromthe group based on the comparison. In essence, the electrodes that aretoo far away from the virtual pole to generate an electrical fieldhaving any significant emulation effect on the virtual pole will beeliminated from the electrode group.

As one example, the function of the distance may be a proportionalfunction (i.e., A₁*(distance), where A₁ is a constant). As anotherexample, the function of the distance may be a higher order powerfunction (i.e., A₁*(distance)^(A2x), where A₁ and A₂ are each constants,and x is an integer equal to 2 or more) or an exponential function(i.e., A₁e^(A) ² ^(*distance), where A₁ and A₂ are each constants). Inthis manner, the exponential decay of the electrical field can be takeninto account when determining which electrodes are too far away to havea significant effect on emulating the virtual pole.

Because an electrical field is directional, the direction of eachelectrode relative to the virtual pole may be relevant (e.g., thehorizontal distance may be more relevant to the vertical distance, orvice versa). In this case, the function of the distance may take intoaccount the x-component (horizontal direction) and y-component (verticaldirection). For example, the function of the distance may be A₁e^(A) ²^(*distance) ^(—) ^(x)*e^(A) ³ ^(*distance) ^(—) ^(y), where A₁, A₂, andA₃ are each constants, distance_x is the x-component of the distancebetween the electrode and the virtual pole, and distance_y is they-component of the distance between the electrode and the virtual pole.As another example, the function of the distance may be A₁e^(A) ²^(*distance) ^(—) ^(x)+A₃e^(A) ⁴ ^(*distance) ^(—) ^(y), where A₁, A₂,A₃, and A₄ are each constants, distance_x is the x-component of thedistance between the electrode and the virtual pole, and distance_y isthe y-component of the distance between the electrode and the virtualpole.

In the examples illustrated above, the function of the distance betweeneach electrode and the relevant virtual pole can be compared to athreshold value, and if the function of the distance is greater than thethreshold value, the electrode will be excluded from the group.Alternatively, the function of the distance between each electrode andthe relevant virtual pole can be an inverse of the functions discussedabove, in which case, the electrode will be excluded from the group ifthe function of the distance is less than the threshold value.

Next, the controller/processor 80 determines a stimulation amplitudedistribution for the electrode group based on the distances between theelectrodes of the group and the virtual pole, thereby emulating thevirtual pole (step 210). Such distances were previously determined instep 206 in order to determine the electrodes to be included in thegroup. In the illustrated embodiment, the stimulation amplitudedistribution is defined as fractionalized electrical current values thatare assigned to the respective electrodes, such that the values for eachpole totals to 100. However, in alternative embodiments, the stimulationamplitude values may be normalized current or voltage values (e.g.,1-10), absolute current or voltage values (e.g., mA or V), etc.Furthermore, the stimulation amplitude values may be parameters that area function of current or voltage, such as charge (currentamplitude×pulse width) or charge injected per second (currentamplitude×pulse width×rate (or period)).

Based on the premise that the further the electrode is from the virtualpole, the less the electrode contributes to emulating the virtual pole,the controller/processor 80 preferably determines the stimulationamplitude distribution by computing weights as decreasing functions ofthe distances, and computing the stimulation amplitude valuesrespectively for the electrode group as a function of the computedweights.

As one example, the decreasing function of the distance may be aninverse proportional function (i.e.,

$\frac{A_{1}}{distance},$

where A₁ is a constant). As another example, the decreasing function ofthe distance may be an inverse higher order power function of thedistance (i.e., A₁*(distance)^(−A) ² ^(x), where A₁ and A₂ are eachconstants, and x is an integer equal to 2 or more) or an inverseexponential function of the distance (i.e., A₁e^(−A) ² ^(*distance),where A₁ and A₂ are each constants). In this manner, the exponentialdecay of the electrical field can be taken into account when determiningthe stimulation amplitude value of the electrode.

As previously discussed, because an electrical field is directional, thedirection of each electrode relative to the virtual pole may berelevant. In this case, the decreasing function of the distance may takeinto account the x-component (horizontal direction) and y-component(vertical direction). For example, the decreasing function of thedistance may be A₁e^(−A) ² ^(*distance) ^(—) ^(x)*e^(−A) ³ ^(*distance)^(—) ^(y), where A₁, A₂, and A₃ are each constants, distance_x is thex-component of the distance between the electrode and the virtual pole,and distance_y is the y-component of the distance between the electrodeand the virtual pole. As another example, the decreasing function of thedistance may be A₁e^(−A) ² ^(*distance) ^(—) ^(x)+A₃e^(−A) ⁴^(*distance) ^(—) ^(y), where A₁, A₂, A₃, and A₄ are each constants,distance_x is the x-component of the distance between the electrode andthe virtual pole, and distance_y is the y-component of the distancebetween the electrode and the virtual pole.

As one example, with reference to FIG. 11 a and 11 b, electrodes E12,E13, E19, and E20 are closest to the central virtual cathode 162 of thevirtual tripole configuration 160, and are selected to be included inthe group of electrodes that will emulate the central virtual cathode162. The distances d₁ and d₂ respectively between electrodes E12 and E19and the central virtual cathode 162 are equal to each other, and thedistances d₃ and d₄ respectively between electrodes E13 and E20 and thecentral virtual cathode 162 are equal to each other, but less than thedistances d₁ and d₂. As such, the stimulation amplitude values forelectrodes E13 and E20 are equal to each other, and the stimulationamplitude values for electrodes E12 and E19 are equal to each other, butless than the stimulation amplitude values for electrodes E13 and E20.

In the case where the stimulation amplitude values are fractionalizedcurrent values, the stimulation amplitude value for each electrode inthe electrode group may be computed as the quotient of the weight forthe electrode divided by the sum of the weights for the electrode group.For example, if the weights for electrodes E12, E13, E19, and E20 are asfollows: w₁₂=2, w₁₃=3, w₁₉=2, w₂₀=3, then the fractionalized electricalcurrent for the electrode group can be computed using the equation:

${{E_{12}(\%)} = {\frac{w_{12}}{w_{12} + w_{13} + w_{19} + w_{20}} = {\frac{2}{2 + 3 + 2 + 3} = {\frac{2}{10} = {20\%}}}}};$${{E_{13}(\%)} = {\frac{w_{13}}{w_{12} + w_{13} + w_{19} + w_{20}} = {\frac{3}{2 + 3 + 2 + 3} = {\frac{3}{10} = {30\%}}}}};$${{E_{19}(\%)} = {\frac{w_{19}}{w_{12} + w_{13} + w_{19} + w_{20}} = {\frac{2}{2 + 3 + 2 + 3} = {\frac{2}{10} = {20\%}}}}};{and}$${E_{20}(\%)} = {\frac{w_{20}}{w_{12} + w_{13} + w_{19} + w_{20}} = {\frac{3}{2 + 3 + 2 + 3} = {\frac{3}{10} = {30{\%.}}}}}$

Notably, if there is only a single virtual pole of a particularpolarization (e.g., the central virtual cathode 162), the totalfractionalized current for each electrode group will equal 100%.However, if there are multiple virtual poles of the same polarization(e.g., the upper virtual cathode 164 and the lower virtual cathode 166),the fractionalized electrical current will all electrode groupsassociated with the same polarized virtual poles will total 100%; thatis, the fractionalized electrical current for each electrode group willtotal less than 100%. In the preferred embodiment, the totalfractionalized electrical current for each electrode group isproportional to the fractionalized electrical current for the virtualpole associated with the electrode group. For example, if 75% of theanodic current is assigned to the upper virtual anode 164, and 25% ofthe anodic current is assigned to the lower virtual anode 166, thefractionalized electrical current for the electrode group (e.g.,electrodes E10, E11, E18, and E19) selected for the upper virtual anode164 will total 75%, and the fractionalized electrical current for theelectrode group (e.g., electrodes E14, E15, E22, and E23) selected forthe lower virtual anode 166 will be 25%.

Once the stimulation amplitude distribution for the electrode group thatemulates the currently selected virtual pole is computed, thecontroller/processor 80 determines whether all of the poles of thevirtual multipole configuration has been emulated (step 212). If all thepoles of the virtual multipole configuration have not been emulated, thecontroller/processor 80 emulates another pole of the virtual multipoleconfiguration 150 (steps 204-210). If all the poles of the virtualmultipole configuration have been emulated, the controller/processor 80conveys stimulation parameters defining the stimulation amplitudedistribution to the IPG 14 via the telemetry circuitry 86 (step 214).The IPG 14 will then convey electrical stimulation energy to the activeelectrodes 26 in accordance with the stimulation parameters. If thevirtual multipole configuration is modified at step 200, the modifiedvirtual multipole configuration may be emulated by repeating steps202-214.

Although the foregoing techniques have been described as beingimplemented in the CP 18, it should be noted that this technique may bealternatively or additionally implemented in the RC 16, and theprocessing functions of the technique can even be performed in the IPG14. Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. A system for an electrical neurostimulatorcoupled to a plurality of electrodes, comprising: a processor configuredfor defining a virtual multipole configuration relative to the pluralityof electrodes, determining a distance between each of a group of theelectrodes and a virtual pole of the virtual multipole configuration,and determining a stimulation amplitude distribution for the electrodegroup based on the determined distances, thereby emulating the virtualmultipole configuration; and a controller configured for instructing theelectrical neurostimulator to convey electrical energy to the electrodegroup in accordance with the computed stimulation amplitudedistribution.
 2. The system of claim 1, wherein the virtual pole is oneof a virtual cathode and a virtual anode.
 3. The system of claim 1,wherein the virtual multipole configuration is one of a virtual bipoleconfiguration and a virtual tripole configuration.
 4. The system ofclaim 1, wherein the processor is further configured for limiting thenumber of electrodes in the electrode group to a maximum number.
 5. Thesystem of claim 4, wherein the processor is further configured forvarying the maximum number of the electrodes in the electrode group inresponse to a user input.
 6. The system of claim 1, wherein theprocessor is further configured for comparing a function of a distancebetween each of the plurality of electrodes and the virtual pole to athreshold value, and excluding the each electrode from the electrodegroup based on the comparison.
 7. The system of claim 6, wherein thefunction of the distance is a proportional function.
 8. The system ofclaim 6, wherein the function of the distance is one of a higher orderpower function and an exponential function.
 9. The system of claim 1,wherein the processor is configured for determining the stimulationamplitude distribution for the electrode group by computing weights asdecreasing functions of the distances, and computing stimulationamplitude values respectively for the electrode group as a function ofthe computed weights.
 10. The system of claim 9, wherein each of theweights is computed as an inverse proportional function of therespective distance.
 11. The system of claim 9, wherein each of theweights is computed as one of an inverse higher order power function ofthe respective distance and an inverse exponential function of therespective distance.
 12. The system of claim 9, wherein the stimulationamplitude values are fractionalized current values, and the processor isconfigured for computing a fractionalized current value for eachelectrode of the electrode group as the quotient of the weight for theeach electrode divided by the sum of the weights for the electrodegroup.
 13. The system of claim 1, further comprising a user interfaceconfigured for generating directional control signals, wherein theprocessor is further configured for modifying the virtual multipoleconfiguration relative to the plurality of electrodes in response to thedirectional control signals, and repeating the distance determinationand stimulation amplitude distribution steps for the modified virtualmultipole configuration, and the controller is further configured forrepeating the electrical energy conveyance step for the modified virtualmultipole configuration.
 14. The system of claim 1, further comprisingtelemetry circuitry, wherein the controller is further configured fortransmitting stimulation parameter sets defining the computedstimulation amplitude distribution to the neurostimulation device viathe telemetry circuitry.
 15. The system of claim 1, further comprising ahousing containing the processor and controller.
 16. A method ofproviding therapy to a patient using a plurality of electrodes implantedwithin the patient, comprising: defining a virtual multipoleconfiguration relative to the plurality of electrodes; determining adistance between each of a group of the electrodes and a virtual pole ofthe virtual multipole configuration; determining a stimulation amplitudedistribution for the electrode group based on the determined distances,thereby emulating the virtual multipole configuration; and conveyingelectrical energy from the electrode group in accordance with thecomputed stimulation amplitude distribution.
 17. The method of claim 16,wherein the virtual pole is one of a virtual cathode and a virtualanode.
 18. The method of claim 16, wherein the virtual multipoleconfiguration is one of a virtual bipole configuration and a virtualtripole configuration.
 19. The method of claim 16, further comprisinglimiting the number of electrodes in the electrode group to a maximumnumber.
 20. The method of claim 19, further comprising varying themaximum number of the electrodes in the electrode group in response to auser input.
 21. The method of claim 16, further comprising comparing afunction of a distance between each of the plurality of electrodes andthe virtual pole to a threshold value, and excluding the each electrodefrom the electrode group based on the comparison.
 22. The method ofclaim 21, wherein the function of the distance is a proportionalfunction.
 23. The method of claim 21, wherein the function of thedistance is one of a higher order power function and an exponentialfunction.
 24. The method of claim 16, wherein determining thestimulation amplitude distribution for the electrode group comprisescomputing weights as decreasing functions of the distances, andcomputing stimulation amplitude values respectively for the electrodegroup as a function of the computed weights.
 25. The method of claim 24,wherein each of the weights is computed as an inverse proportionalfunction of the respective distance.
 26. The method of claim 24, whereineach of the weights is computed as one of an inverse higher order powerfunction of the respective distance and an inverse exponential functionof the respective distance.
 27. The method of claim 24, wherein thestimulation amplitude values are fractionalized current values, and thefractionalized current value for each electrode of the electrode groupis computed as the quotient of the weight for the each electrode dividedby the sum of the weights for the electrode group.
 28. The method ofclaim 16, further comprising: generating directional control signals;modifying the virtual multipole configuration relative to the pluralityof electrodes in response to the directional control signals; andrepeating the distance determination, stimulation amplitudedistribution, and electrical energy conveyance steps for the modifiedvirtual multipole configuration.